There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
CMOS imagers are well known. CMOS images are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366, 6,326,652, 6,333,205, and 6,326,868; assigned to Micron Technology, Inc., the entire disclosures of which are incorporated herein by reference.
Solid state imaging devices include an array of pixel cells, which converts light energy received, through an optical lens, into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing the pixel cell charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
To detect color, the spectral components of incident light must be separated and collected. An absorptive color filter array (CFA) on top of an imager chip is currently the dominant technology for color detection in a solid state imager, for example, a CCD or CMOS imager. In a typical imager layout, a micro-lens and CFA is stacked as part of a pixel stack. In an effort to reduce the pixel stack height and bring the micro-lens and CFA closer to the photosensor, the CFA can be lowered into a recessed area within the imager. However, the problem exists that if the recess depth exceeds the thickness of the CFA film, the typical method of planarizing, i.e., chemical mechanical planarization (CMP), is no longer directly applicable to improve the planarity of the CFA on the pixel array.
There is needed, therefore, another method and apparatus providing a uniform color filter array within a recessed area in an imager for situations when the recessed area depth exceeds the thickness of the CFA film and improving the planarity of a recessed CFA.